Ceramic membranes are widely used in microfiltration and ultrafiltration. This is due to a number of advantages that they have over polymer counterparts. The advantages include a greater mechanical strength and structural stiffness, greater corrosive and thermal resistance, stable operating characteristics during long service, and the possibility of multiple regenerations by calcination or by the backward stream of water or an appropriate solvent. This means that ceramic membranes can be operated over a wide pH range, at high temperatures and pressures, and in corrosive media. On the other hand, these membranes can be brittle and also expensive due to the energy-intensive technology of their fabrication.
Ceramic membranes are of interest for filtration systems, such as for the filtration of water where the high strength material allows for the use of high pressure filtration. Examples of such filters are discussed in US2006/0175256.
The separation performance of filtration membranes is normally evaluated by two factors, the selectivity and the permeability. In a porous ceramic filter, selectivity is determined by membrane pore size formed by the voids between packed particles or grains after the membrane is formed. Permeability is a function of pore size, pore size distribution and porosity. In order to achieve desired pore size and high permeability, a multilayer membrane structure may be employed, in which one or more layers of finer particles are progressively coated or deposited onto another substrate layer made of bigger particles, forming a gradient pore structure.
As discussed in “A morphological study of hollow fiber membranes”, Kingsbury et al., Journal of Membrane Science 328 (2009) 134-140, it is possible to prepare ceramic hollow fiber membranes by a method of phase inversion. Such membranes have a good porous structure and are ideal for use at high temperatures and pressures, and in corrosive environments. However, it has been found that these methods do not necessarily provide sufficiently high permeability for use in all desired applications.